Roll-off vehicles are known to include a top frame mounted on the vehicle for supporting the container. The top frame is pivotally mounted to be tilted away from a horizontal home position such that the top frame is inclined toward a rear of the vehicle to assist in loading and unloading the container. The top frame may be tilted by one or more lift actuators, for example hydraulically powered linear actuators, arranged between the top frame and a subframe fixed to the vehicle. The top frame may incorporate a cable hoist mechanism having a set of sheaves and a cable wound around the sheaves. Some of the sheaves may be mounted on one or more movable shoe members that are displaceable along the top frame to take up an end of the cable coupled to the container to pull the container onto the top frame during loading. Sheave displacement may be reversed to unload the container. A locking mechanism may be provided at the front of the frame for engaging a to secure the
U.S. Pat. No. 7,192,239 to Marmur et al. discloses a roll-off frame equipped with a cable hoist mechanism having a front sheave and a pair of rear sheaves at fixed locations on the frame, and two intermediate sheaves carried by respective movable shoe members for displacement along the frame relative to the fixed front and rear sheaves. A linear actuator is operable to displace a primary shoe member during a first stage while a secondary shoe member remains stationary. During a second stage, the primary shoe member abuts with the secondary shoe member such that further displacement of the primary shoe causes corresponding displacement of the secondary shoe member. The first stage provides enhanced power and slow speed as the container is initially pulled forward, and the second stage provides greater speed and less power.
U.S. Pat. No. 8,029,228 to Marmur discloses a roll-off frame in which the cable hoist mechanism includes a single movable shoe member for displacing a multi-grooved sheave relative to a pair of sheaves mounted at a fixed forward location on the frame. A locking mechanism is provided near the front of the frame to secure front rollers or a lateral member of the container to hold the container in position on the frame. The locking mechanism includes a pair of laterally spaced front hook members on a top frame portion arranged to receive the front rollers or lateral member of the container when the container is pulled fully forward onto the top frame portion, and a pair of laterally spaced locking struts fixed to a subframe portion and positioned to block the hook openings when the top frame is pivoted down to a horizontal home position with respect to the subframe.
The prior art roll-off frames taught by the aforementioned U.S. Pat. Nos. 7,192,239 and 8,029,228 are well-suited for handling containers of a particular length for which the roll-off frame is designed. However, if these roll-off frames are used to load and unload containers of different lengths, the lift actuators may be subjected to unintended loading conditions caused by differences in the location of the center of gravity of the container and its contents.
U.S. Pat. No. 4,840,532 to Galbreath teaches a “universal” roll-off frame intended for use with containers of various lengths. The roll-off frame is characterized by a two-part top frame having a main portion and a forward head portion telescopically extendable and retractable relative to the main portion to adjust the overall length of the top frame. The roll-off frame of Galbreath incorporates a cable hoist mechanism having a first pair of laterally spaced sheaves mounted on a movable shoe member that slides along the main portion of the top frame, a second pair of laterally spaced sheaves mounted at a fixed location at a front end of the main portion, and a front sheave mounted at a front end of the extendable head portion of the frame. A pair of hydraulic linear actuators are mounted to displace the movable shoe member to change the longitudinal distance between first pair of sheaves and the second pair of sheaves. A third hydraulic linear actuator is arranged to extend and retract the head portion of the top frame relative to the main portion of the top frame. An end of the hoist cable is attached to the main portion of the top frame, and the cable is wound around the sheave pairs and the front sheave.
Before using the roll-off frame of Galbreath, the operator must adjust the head portion relative to the main portion to achieve a desired overall length for a given container, and releasably fix the head portion in position to prevent further movement of the head portion relative to the main portion. For this purpose, a series of longitudinally spaced holes are provided along lateral sides of the main portion and head portion, and shot pin actuator is operable to insert a pair of pins into aligned holes on each side of the top frame to lock the head portion in position. Thus, in Galbreath, the length of the top frame is adjustable to one of a plurality of discrete length settings prior to use. The length of the top frame in Galbreath does not adjust during loading or unloading of a container. The hydraulic actuator for extending and retracting the head portion does not contribute power to the cable hoist mechanism. Galbreath has a total of five hydraulic actuators: two lift actuators for tilting the top frame, two actuators for displacing the shoe member, and one actuator for extending and retracting the head portion of the top frame. Consequently, Galbreath requires a complex hydraulic circuit.
Galbreath also describes a locking mechanism for securing the container on the top frame. The locking mechanism of Galbreath includes a pair of laterally spaced front hook members on a top frame portion arranged to receive the front rollers or lateral member of the container, and a pair of laterally spaced spring-loaded detents adjacent the hook members. The detents pivot downward against the spring bias as the container is pulled into its fully forward position, and pivot back up once the container is in place.
The locking mechanisms described in U.S. Pat. No. 8,029,228 and in the patent to Galbreath have drawbacks. The locking mechanism in U.S. Pat. No. 8,029,228 is useful where the top frame of the apparatus has a fixed length with no extendable head. However, if an extendable head is provided, the use of locking struts fixed to the subframe is precluded because the position of the front hooks on the extendable head will change with respect to the locking struts fixed to the subframe. The locking mechanism in Galbreath overcomes this problem, but does so using a spring mechanism that may become jammed with exposure to moisture, dirt and debris. If jamming occurs, the operator may be unaware that the detents have failed to spring up into blocking position.